Cellulosic substrates having a surface coating thereon such as an adhesive-coated cellulosic paper are well-known to the prior art (see U.S. Pat. No. 3,055,496 to Dunlap, and U.S. Pat. No. 3,677,788 to Zirnite). Control of the depth of penetration of the coating material into the interstices of the cellulosic substrate is extremely important since penetration by excessive amounts of coating provides a product which is not economical to manufacture. Another problem which also occurs is strike-through, wherein the coating compound penetrates through the cellulosic substrate causing an unwanted layer of adhesive to be deposited on the coating equipment. This, in turn, results in more frequent equipment shutdown for cleaning purposes.
The ability to prevent excessive penetration of the coating material and to maintain a uniform, continuous, smooth coating surface is defined as "hold-out". The term "hold-out" also encompasses the notion that penetration of the coating composition to the base substrate must be minimized while maintaining efficient coating usage at reasonable coating weights.
In order to facilitate effective hold-out of the coating material, the prior art contemplates treatment of the surface of the cellulosic substrate in a size press with sizing agents such as starch and/or petroleum-based polymers such as styrenemaleic anhydride. Expensive wet-end chemicals are also employed to increase hold-out properties. However, the problem occurs in that many paper machines are not equipped with size presses since this equipment is quite costly to purchase and is often difficult to economically justify. Economic justification for this equipment is of particular difficulty in the case of older paper machines which exhibit more marginal economic benefit to a given manufacturer.